1. Field of the Invention
The present invention relates to an optical recording medium where information can be recorded and reproduced by light beam irradiation.
2. Description of the Prior Art
Optical recording media for record and reproduction of information are roughly classified into three types: a reproduction-only type, postscript (direct read after write) type, and rewriting type. The reproduction-only type devices are used exclusively for reproduction, including video disk and digital audio disk (DAD) devices. The postscript (DRAW) type devices are those in which a blank is allowed to remain after recording and used for additional recording as required. The rewriting type devices permit erasing the written unnecessary information and rewriting other information.
There are known optical recording media, for example, those having a rare earth-transition metal alloy film, those based on an amorphous-to-crystalline phase transition which have a film of reducible oxide such as a chalcogen compound, heat-mode recording media, and thermoplastic recording media. Among these, representative media in which rewriting is possible include those having an optomagnetic recording layer made of a rare earth-transition metal alloy. Materials usable for the optomagnetic recording layer include, for example, polycrystalline films of MnBi and MnCuBi, amorphous films of GdCo, GeFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo, TbFeCo, and GdTbCo, and a single-crystal film of GIG (Gadolinium-Iron garnet).
The above optical recording media are generally provided with guide tracks for a tracking servo adjacent to recording tracks so as to track the recording tracks exactly during recording, reproducing, and erasing.
A technique for forming these guide tracks in optical recording media has been proposed, which comprises, as shown in FIG. 1, covering a groove-engraved plastic substrate 1 with an optomagnetic recording layer 2 to form guide tracks 2a and recording tracks 2b. This technique, however, is disadvantageous in that the S/N ratio is much lowered under the influence of the birefringence due to the substrate 1. Moreover, the memory density is low because the grooves are formed mechanically and when the substrate 1 is made of glass, the groove engraving requires complicated operations and a high cost.
Besides this technique, Japanese Pat. Kokai Nos. 61031/81, 61032/81, and 74854/82 have disclosed techniques which comprise, as shown in FIG. 2, forming guide tracks 2a by irradiating the optomagnetic recording layer 2 made of an amorphous magnetic film with a high power laser beam to crystallize the irradiated portions or convert them into longitudinally magnetized layers. However, it is disadvantageous that a high power laser beam is necessary to transform the amorphous magnetic film.